The primary purpose of lubrication is separation of solid surfaces moving one relative to another, to minimize friction and wear. Solids and fluids (gases as well as liquids) are employed as lubricants. The materials most frequently used for this purpose are oils and greases. The choice of lubricant is mostly determined by the particular application.
Lubricating greases are employed where heavy pressures exist, where oil drip from the bearings is undesirable or where the motion of the contacting surfaces is discontinuous so that it is difficult to maintain a separating film in the bearing. Because of design simplicity, decreased sealing requirements and less need for maintenance, greases are almost universally given first consideration for lubricating ball and roller bearings in electric motors, household appliances, automotive wheel bearings, machine tools or aircraft accessories. Greases are also used for the lubrication of small gear drives and for many slow-speed sliding applications.
Lubricating greases consist primarily of a fluid lubricant, such as an oil, and a thickener. Essentially, the same type of oil is employed in compounding a grease as would normally be selected for oil lubrication. Of various available oils, petroleum oils are used most frequently, but many other natural or synthetic oils are employed to provide desirable physical and chemical properties of the produced greases. Fatty acid soaps of lithium, calcium, sodium, aluminum and barium are most commonly used as gelling agents. However, thickeners may be one of a variety of solid materials, including clays and pigments. Of these, clay-thickened greases are characterized by high dropping points, frequently higher than 260.degree. C.
Various chemical additives are incorporated into lubricating greases, to impart certain desirable characteristics to the grease, such as oxidation stability, tackiness, extreme pressure properties and corrosion inhibition. Solid materials such as graphite, molybdenum disulfide, talc, metal powders, and various polymers may also be added to impart special properties.
Dyes or pigments are occasionally added to lubricating oils and greases. U.S. Pat. No. 4,764,290 discloses a method of providing identification to oil by dispersing in the oil particles of a hydrophilic dye which do not substantially change the color of the oil. When a sample of the oil is exposed to a polar solvent, the dye particles dissolve in the solvent providing a perceptible, intensive color. A similar identification system for non-water miscible organic liquids, including lubricating oils, is disclosed in U.S. Pat. Nos. 3,764,273 and 3,883,568.
Dyes or pigments may also be added to impart a distinct coloration to greases. Canadian Patent No. 626,227 discloses grease compositions containing as major gelling agent clays containing strongly adsorbed dyes. The greases so produced possess a characteristic identifying color which is additionally advantageous for determining if sufficient quantities of grease are present on the lubricated bearings.
Dyes or pigments may be added to lubricating compositions as detectors of unfavorable operating conditions. For example, Japanese Patent No. 88004600 discloses a water-detecting grease composition prepared by dispersing in the grease water soluble dyestuffs or pigments which exhibit a different color in the presence of water. The grease becomes distinctly colored when mixed with a small amount of water, so that the presence of water in the grease can be easily detected. This may be useful, for example, for checking sealing conditions of a lubricated part.
In mechanical equipment temperature is one of the most important operating parameters of its lubricated parts, such as bearings. The operation of the equipment results in an increase of temperature of the lubricated parts. For a given part, there is normally a maximum temperature which should not be exceeded under normal operating conditions. However, the maximum operating temperature may be exceeded under extreme operating conditions as a result of high ambient temperatures or mechanical malfunctions. Unless the temperature of the lubricated element is continuously monitored, such occasional or permanent overheating is likely to remain unnoticed, which may result in reduced lubricant effectiveness and potentially in irreversible damage to the equipment. It is therefore desirable to provide a simple method of detecting whether a predetermined limit temperature of the lubricated element has been exceeded during its operation.